A contributing factor to a wide spectrum of human diseases is defective cell polarity. Many cell types use mRNA localization as a means to establish polarity. At present, there is a fundamental gap in our understanding of how mRNAs are localized in higher eukaryotes. The goal of this proposal is to address this gap in knowledge by determining the mechanism by which oskar and FMR1 mRNAs are localized. oskar mRNA is localized in Drosophila oocytes and FMR1 mRNA is localized in neurons. These transcripts are ideal candidates for study because although they are localized in different tissues, their localization utilizes many of the same factors. Thus, a common mechanism might operate to localize diverse mRNAs. Insights gained from these studies are therefore expected to be generally applicable to our understanding of mRNA localization. We propose to perform our studies using the Drosophila melanogaster model system. This model system will enable us to study the process of mRNA localization in the context of the whole organism. This study has three main objectives. The impetus for Aim 1 was our recent discovery that core proteins of the spliceosome known as Sm proteins have a role, outside of splicing, in mRNA localization. In the present study, we propose to determine the mechanism by which Sm proteins function in localizing oskar mRNA. In Aim 2, we propose to determine the mechanism by which oskar and FMR1 mRNA are coupled to motor proteins for transport. The primary motor implicated in transporting both mRNAs is Kinesin heavy chain (Khc). However, Khc does not use its canonical adaptor to bind to oskar and FMR1 mRNA. Thus, it is currently unknown how Khc binds to these transcripts. We hypothesize that an unknown adaptor links Khc to oskar and FMR1 mRNA. The goal of Aim 2 is to identify this unknown adaptor. Finally, Aim 3 centers on our unexpected finding that a separate motor, cytoplasmic Dynein, also functions in oskar mRNA localization. A function for Dynein in the localization of FMR1 mRNA has already been demonstrated. In this aim, we will precisely define the role of Dynein in transporting oskar mRNA. Furthermore, we will test the hypothesis that once Dynein delivers oskar mRNA to the posterior pole, it performs an additional function in maintaining asymmetric endocytic activity within the oocyte. Completion of these aims will significantly advance our understanding of how the cell builds a localization competent mRNP, and the mechanism by which that mRNP is coupled to motor proteins for transport. Ultimately, the knowledge gained from these studies will enable us to more effectively treat diseases that result from defective mRNA localization and cell polarity.